Handover decisions for mobile terminals traveling through a wireless network are typically made based on factors such as cell coverage, mobile terminal location and terminal movement information. Mobile terminals include a variety of electronic devices, including cellular phones, mobile digital video broadcast (DVB) receivers, pagers, personal digital assistants, laptop computers, automobile computers, portable video players, and other devices which may move among multiple cells and which include equipment for receiving signals from a wireless network. In addition to DVB receivers, mobile terminals may include mobile receivers of other digital unidirectional broadband broadcast systems.
With a first conventional approach, handover decisions are based on location, cell coverage area and terminal movement vector information. The mobile terminal has means to know its exact location (e.g., GPS, AGPS) and measure the signal strength from available signals. FIG. 1 shows a mobile terminal 101 in the crossroads of cell signals A, B, and C, each of which is depicted by a border showing the outer reaches of a single minimum signal level. The movement and velocity of mobile terminal 101 is described by vector 102. Mobile terminal 101 is able to receive any of signals A, B, and C. If mobile terminal 101 is presently receiving either signals B or C, then it executes a handover to signal A in order to continue reception of a signal in the future, assuming the same course and speed.
FIG. 2 depicts a spherical rectangle 201 representation of cell signal A as used by some technical specifications, such as the Digital Video Broadcast (DVB) system. For cell signal A, spherical rectangle has a reference corner 202, typically located at the southwest corner of the rectangle. Reference corner 202 is specified by a specific longitude and latitude, or some other geographic designation. The extent of longitude 203 and the extent of latitude 204 describe the length and height of the bounding rectangle, which is sized to encompass the cell signal. The values associated with extents 203 and 204 are in the form of degrees, minutes and seconds, or spherical or planar vectors, or some other representation having a magnitude. While permitting relatively simple handoff calculations, the use of spherical rectangles is likely to be fraught with inaccuracies.
FIG. 3 illustrates how the cell signals of FIG. 1 may be modeled using the conventional approach depicted in FIG. 2. The cells are assumed to provide the same signal strength within rectangular areas. Based on information provided to mobile terminal 101, the mobile terminal will either perform a handover to cell B, or keep the signal of cell B if already active. Given the signal strength of cell B in FIG. 1, such a determination is poor because of inaccurate cell signal representation, and a signal may be lost.
FIG. 4 presents a more realistic depiction of signal strength using different shades to represent the varying strength and weakness of a cell signal within coverage areas. Although only a few shades are used to represent signal strengths, the infinite range of signal strengths varies depending on environmental conditions within the coverage area and other factors. Under a conventional approach, mobile terminal 101 will make a poor assumption selecting cell C as a handover destination from cell B. Although cell C fully encompasses mobile terminal 101 moving along vector 102, the signal strength will decay if the mobile terminal maintains a connection to cell C. If the reception sensitivity of mobile terminal 101 were to be taken into account, the optimal choice in a handover situation would be to cell A, based on actual signal strengths. It should be noted that the Applicant is not suggesting that varying levels of signal strength have been used in conjunction with prior art handover procedures.
With a second conventional approach, handover decisions are made based on a location determination that is estimated using signal strength information and cell coverage information. With this approach, a mobile terminal is not aware of its location (e.g., doesn't have a GPS system). FIG. 5 illustrates an example of this basic method utilizing only cell shape information (i.e., only one signal level used). Here, because mobile terminal 101 is able to detect signals from both cell A and cell C, it is able to determine that it is somewhere within shaded region 501. FIG. 6 depicts a similar method to detect approximate location using spherical rectangles. Here again, because signals from both cell A and cell C are detected, mobile terminal 101 is able to determine that it is somewhere within shaded area 601. Either method, while facilitating handover decisions, do so in a highly inaccurate manner, since the precise location within the shaded region is unknown.
Therefore, there is a need in the art for handover procedures and systems that utilize accurate models of cell station signal strengths in order to expand handover procedures. In addition, there is a need for methods and systems that share cell station models and other cell coverage information.